Researchers Caitlin Griffith and Ralph Lorenz"Titan gives us a different experiment in the lab of the Solar System" real 28k

Thursday, 19 October, 2000, 19:26 GMT 20:26 UK

And now the weather on Titan...

The Huygens probe will reach Titan in 2004

By BBC News Online science editor Dr David Whitehouse

It may rain droplets of methane on Saturn's largest moon.

New observations of Titan made in the infrared region of the spectrum have detected clouds of methane gas forming and dissipating in the upper reaches of its thick atmosphere.

The clouds lasted about two hours and then faded away, probably because they condensed and showered the surface with methane rain.

Astronomers believe that Titan's atmosphere resembles Earth's, with clouds and rain. There may even be seas on the satellite's surface. They will find out more in 2004 when the European-made Huygens spacecraft plunges into the moon's atmosphere.

Methane, methane everywhere

Titan is the only object in the Solar System with an atmosphere of comparable thickness and overall composition to that of our planet. Like Earth, its atmosphere is mostly nitrogen. It is very cold, however, with a temperature of about -180C.

Titan as seen from the Voyager spaceprobe in 1979

Scientists speculate that instead of having a water cycle like our planet, a methane cycle would be the key component of Titan's weather.

If Titan's atmosphere is supersaturated, as they believe, the methane would condense into high-flying clouds. Precipitation from the clouds could lead to enormous raindrops that would fall at a snowflake-like pace through the thick atmosphere.

Looked at in most regions of the spectrum, Titan's atmosphere is opaque. But there are some narrow wavebands that allow a deeper view into its atmosphere if not to the surface itself.

Closer view

Caitlin Griffith, of Northern Arizona University, Flagstaff, US, and colleagues have obtained spectra of near-infrared light emitted by Titan. They found variations at certain wavelengths that indicate the presence of small clouds.

Observations made over several days suggest that the clouds form and dissipate on a daily basis. This suggests that they are showering rain down on to Titan's surface.

The authors propose that latent heat radiating from the moon, rather than incoming heat from the Sun, may drive a methane-based cycle on Titan similar to the hydrological cycle on Earth.

The Cassini spacecraft is currently en route to Saturn. When it gets there in 2004, a small probe called Huygens will parachute through Titan's haze with the aim of landing on a methane snowdrift or into a methane ocean.